View Full Version : A dilema in vacuum forming

12-06-2006, 08:36 PM
I hav e an old 22x22 Vacuum unit w/overhead heat and a 10 gallon tank, pump sucks in at about 25lbs.
I am using .020/.040 petg.
I am trying to form a lens, ( pic attached ) the depth of the valleys between the lines of the lens is about .007".
I am getting the petg to form around the lens plate, but not the lens themselves. i.e the petg is not forming in the valleys, I get a flat inner surface. I am very open to idea's does anyone have any.
Thanks Gary[/img]
Well I guess I'm not dropping a pic!
the plate is 12x12" it has 30 convex lines per inch. my vacuum table has holes every 2"[/list]

12-06-2006, 08:53 PM
If there is no venting at the bottom of the valleys the plastic will chill off long before enough vacuum will be pulled in this area. With 40 mil matrerial you could get by with .032 drill bits. but with it being a lense construct you would probably want to vent with a drill in the 70 to 80 regions. Another thing to consider is that pet likes hot molds. If you want super detail you are going to have to pressure form.


12-06-2006, 09:10 PM
Drilling is out of the question! I thought of pressure forming, I picked up a t-shirt heat transfer machine! It works great around the sides but, the petg's flat as a pancake in the centre ( the hot platern is slightly concave ) I agree with you John it does need pressure, thanks mate

12-06-2006, 09:27 PM
What kind of "lens" is this? How fine is the detail that you'd need for the optical properties you want? (E.g., do you need 1/30" ridges to show up a little, or do you need the planes between them to be distinct and flat?)

12-06-2006, 10:17 PM
Actually, there are no planes only valleys with convex ridges and they have to be as precise as possible to allow the lens to focus. I have tried in the past the method with the t-shirt machine. It was not the greatest, there were quite a few imperfections + I had wiped on silicone to the plate and a silicone sheet under the petg which affected the clarity as the silicone left a dull film. Also i had to wait quite a while for the brass plate to cool down before removing the petg.
If you have any idea's for vacuum forming other than drilling the plate or, any other process, or idea's I'm all ears.
Thanks for your response GAry

12-06-2006, 10:44 PM
For the kind of fine detail and precise angles you need for a focusing Fresnel, I wouldn't think vacuum forming would be the way to go. A silicone mold and clear resin casting would seem much better.

12-06-2006, 10:54 PM
And then there's the issue of refractive index... if you make the copy out of something with a different refractive index than the original, it will not focus the same way even if you do get the shape right. Depending on what kind of image you need, it might more or less work with a different focal length, but it's likely to be useless.

What kind of lens are we talking about? I'm not asking for any proprietary detail, just general parameters. Is it something like the Fresnel in an overhead projector?

One of Many
12-07-2006, 02:33 AM
I take it the die is a dish shape? Are the ribs toward or away from the die? Does the vac plate close off the holes outside the blanks 12x12 area?

Even if you pressure form it, just as in vacuum, the dish will trap air if no way for it to escape. one small hole if .010-.015 would not be noticable if you can get your pressure or vacuum time and heat under control.

Another method is compression forming after heating. For a lens like yours, I would line the upper die with felt to keep from distorting the ribbed surfaces while id forces the material to conform into the lower die.

With such a fine rib, the heat can level out those ribs too?


12-07-2006, 10:16 AM
Firstly, the plate is for a lenticular lens, Dr you are quite right the female has to be a total duplicate of the male as well as a specific thickness. The plate is flat DC, and there is no upper die.
The input I am getting is great, I'm from the school that, it's not what I read, it's how I read into it.
I'm going to try two methods.

a. I am going to place (in pic) a pc of aluminium over the two ends and coveing a line of holes, my hopes are that I will have more pull from that area. Rolling Stones, You can't always get what you want but, if you try some time you just might find you get what you need, I hope.

b. Attaching the plate to the bottom of the heat transfer platen (this will cover the convex curvature of the platern) and not use the silicone, I'll try a teflon sheet under the petg. I am hoping that, if the male plate being brass will allow a release of the petg when it has cooled down
Any thoughts?

12-07-2006, 12:00 PM
Interesting problem.

The lenticular lens itself is curved overall? It's not one of those flat ones with all the grooves straight and parallel?

For a flat one, you could try using your vacuum former as a pressure former, by putting an oversized rigid plate over the hot plastic, and sucking that down to squeeze the plastic. All of the force from sucking down the whole plate would be transferred to the plastic. (If your rig isn't sturdy enough, though, you could break your platen.) You'd want to make sure the middle got squeezed a bit sooner than the edges, to push the air outward through the grooves as the plate comes down.

12-07-2006, 12:30 PM
The lenticular lens does have straight and parallel grooves only they are not like triangles, they have convex tops.
Okay, let's take that into consideration!Let's say I place this plate on top of the hot plastic acoule of points have to be taken into consideration.
a. The plate will cool down the plastic fast, if so, will that effect the forming.
b. I have a problem vacuuming the top plate as the vacuum holes do not lay outside the holding frame.
What I have found that is giving me a better pull into the orifices of the lens, is the laying and sealing of the aluminium strips at either end of the father plate.
I am going to try blocking a few more holes around the plate, this will concentrate more suction into the top of the plate. I am also going to try your suggestion by adding a heavy plate with a wooden base (won't be a cold as metal)
As far as being sturdy enough, I believe this baby was made in the 30's solid iron, it takes 4 blokes to lift it.

One of Many
12-07-2006, 02:48 PM
I understand there is no upper die now. That would be in compression forming the lens as an option.

As I read it, the die is a concave/convex depression and that you have no vac holes inside this concave area. I take that to mean, once the material seals off around the rim, it is much less likely the material will pull down into the concave.

If any of the holes in your platten suck air during the forming process, you won't get full vacuum under the lens. In a sealed cavity, vacuum, just like pressure is equal on all surfaces. You cannot get the vacuum to suck harder in one particular area.

Unless you are trying to prevent leakage in the die area by using the bars you plan to add to give the sheet something to seal against. The platten with the vac holes in it should still be sealed off with the exception of what is directly under the forming area. Any external leakage is the path to least resistance, leaving losses if nothing pulling on the plastic itself.

There is also a method that utilizes a seal ring. Pressing on the material, just outside the trim line to hold the material to the tool. Nothing more than a non cutting cookie cutter that stomps down on top of the material sealing it against the die prior to applying vacuum. It prevents leakage at the material edge if the material is not held tight in a clamping frame. This precludes that vacuum must be ported to the die cavity only.

I'd need to see the die, your process and forming machine to offer anything of further use to help resolve your dilema.


12-07-2006, 05:38 PM
All of the platen holes outside the area the plastic covers should be sealed off---or, if you're using a bigger plate, there should be a compressible gasket of some sort to seal to the platen around the edge. Any platen air holes further out should be taped over or otherwise sealed off, to keep the vacuum in.

A more detailed description of the shape would be very helpful. Is it flat or domed overall? If it's domed, how much is it domed, over what distance?

I suggested a rigid metal plate just to distribute the force evenly over the plastic during forming. That amounts to a flat die for the non-grooved side. I'm guessing you want that surface smooth; a wooden plate might be a lot of work to get smooth enough.

I wouldn't be surprised if a steel plate worked fine; if the PETG or whatever plastic you're forming is thick enough, it will insulate the grooved side of the plastic long enough to form. (This should all happen very quickly.) On the other hand, maybe not, and maybe it'd be bad to cool the plastic that fast on one side. You could cover the flat die (plate) with a flat piece of some other plastic, preferably with a higher melting point. For a metal plate, that would provide some insulation. For a wooden one, it'd provide a smooth surface.

This is harder if your shape is domed, of course. If so, you can could cast a domed adapter to match, off the non-grooved side of the lens you're copying.

Either way, I'm wondering if you can get good enough detail for a lenticular lens. Even assuming you squeeze the air out well through the grooves (and maybe some tiny holes), I'd guess you'll get enough rounding of the edges of the grooves (the edges of the flat refracting strips) that it will blend the images it's supposed to keep separate, at least somewhat, and likely have other optical flaws.

What kind of lenticular lens is this? Is it for a 2 image thing where you see one image if you're off-center one way, and another if you're centered or off-center the other way?


12-07-2006, 05:41 PM
Also, how many of these things are you making? If it's only a few, would you be better off casting it in resin?

You may need to cast it anyway, to get a buck you can drill vent holes in. And if you're only making a few, it probably makes more sense to just cast the parts conventionally.

12-07-2006, 05:46 PM
I have enclosed a pic of lens.
I take that to mean, once the material seals off around the rim, it is much less likely the material will pull down into the concave. .
Hmmm Not necessarily and I'll try and cut to the chase.If when the petg is brought down it is held above the lens, then sucked onto the lens, which have valleys (also the middle sinks faster than the sides). Then logically it sb pulled down centre first.??
I agree You cannot get the vacuum to suck harder in one particular area. But, what I can get is a longer stronger pull in a smaller area.
Now I tried Dr's suggestion of adding weight only, I did it after the vacuum, now what I am getting from you is, to add the weight prior to or as close as possible to suction OK
Also when I tried the weight on after vacuum it had very little effect as the petg had already cooled.
As I mentioned earlier I am half way into the valley.
I'll get pics of machine tomorrow.

One of Many
12-08-2006, 01:26 AM
If when the petg is brought down it is held above the lens, then sucked onto the lens, which have valleys (also the middle sinks faster than the sides). Then logically it sb pulled down centre first.??

I'll look forward to the picture.

So I think it is becoming clear.....

You are expecting the die with ribs in it to form the properties of a lenticular lens. I would think you will end up with a corregated sheet of PETG instead of a lens, but that is pure conjecture on my part. I do not see the back side remaining optically flat. You might have better luck displacing the material similar to roll forming threads or knurling, I. E. Embossing with a hydraulic press between two heated plates. I am not sure what would result since shrinkage in the plastic can make it a challenge to keep it flat after removing it from the grooved die plate.

Just to see if you could get it to pull tight. I'd try forming directly on the grooved die plate itself. The only change I would make would be to place a snug fitting square ring around the 12x12 die, the same hieght as the die. Seal the outside of the ring to your table so that vacuum is only pulled through the a very small gap between the outside of the die and inside of the ring. If you had to, you could place the die on top of thin screen, shims etc to prevent restrictions to the vacuum holes below the die. You could also cut small grooves into the side faces of the ring or die to port vacuum to each of the grooves on your die plate. This should create a natural path so vacuum will pull the material into the grooves of the die plate with minimal effort if the material is hot enough. If need be use a heat gun to get and keep the die hot so it does not chill off the plastic before it has a chance to form.

Once the material is done heating(preferably in a frame), drape it completely over the ring and die, then hit the vacuum. This must be done quickly. Fine detail is best obtained where the material is pulled tight as a product of proper heat in both the material itself and the die forming temp, then minimum time from heating to point of forming seal and then vacuum. This can be very critical on thin material, but there will always be some limitation based on material characteristics and thickness, not to mention equipment. As the material forms, the surface area the vacuum is acting on gets smaller, it cannot pull it tighter besides the fact it is returning to its solid form rapidly.

Once it is formed on the die, you can turn a fan on it and let it cool for a few minutes, but not too long. When the sheet returns to rigid should be adequate, but it still may be hot to bare skin.

Do you realize that you can buy PETG in sheet with the lenticular aspect extruded in. Spartech Lenticular Lens Sheet (http://www.spartech.com/plastics/lenticular_sheet.html)

Sorry for the confusion. For some strange reason, I thought you were trying to form this as pre-made lens sheet to a specific shape on a female die.


12-08-2006, 06:00 AM
Does it have to be Petg? Have you tried Lexan?. I went threw the thread and I'm a little cornfused on what you are forming but I only use Petg for testing because it's cheap . If I could see a picture of your machine it would help. I think you said 25 lbs which is ok but how big is the tank? Also are you heating the mold? and how far are you saging? and why am I asking so many questions? LOL

12-08-2006, 10:09 PM
Hey Ringo keep asking, the more questions creates more potential for answers!
Well, are you ready for this one. Even though I replaced the Temp controller and the thermo couple, I am having problems with getting above 230f. (I borrowed a temperature gauge) Now if that's the case, is it possible that even though the petg is sagging, and feels hot to the touch it's not soft enough to form such fine lines.

if so, now that I added the aluminium strips to the ends of the father plate, and I am getting half the depth of the lens forming on the petg, It is a possibilty that with more heat I can get this to work.
My dilema on this is that the bloke who was helping me set the controller told me to make a few adjustments, let it run for 20 minutes and then call him back. When I called he had left for the weekend grrr.

2nd method with heat transfer machine, I had manufacturer get me a me a new hot plate, it was perfectly flat, (made my day) my problem with this set up is I have to place something between the hot platen and the petg, I used a silicone sheet, then a telon sheet. I did get a good emboss only the sheet became somewhat opaque. I'm not sure if it was the petg and heat or the after effects from using the sheets, anyone have any thoughts?

It does not have to be petg, I have some pvc coming in next week.
Pics attached.
I am not preheating the mold!
As far as the sagging, did I tell you about this girl I knew. No really about 1"-2".

You too mention heating the mold also, we both agree on a tighter area around the mold. I like the mesh under the die idea. As far as the grooves I believe that has been accomplished by laying the alluminium strips along the end, the alluminium lays on top of the lens leaving the valleys open for the pull. Heat gun hmm yes.
Yes I do know that there are sheets out there only, as Paul mentioned the mother plate (petg) must be a perfect reverse of the father plate (mold).

Hey Paul, I am making a couple of dozen some in hard plastic and some in soft pvc. I do not know how to cast, I am always open to learning only, time is of the essence.

I am sorry I cannot be explicit! I am working on a patent and you blokes are being real helpful, I really appreciate your interest help and patience

12-08-2006, 11:27 PM
When I talked about a plate over the top, I wasn't suggesting a heavy plate. I was suggesting a plate to broaden the area that the vacuum former sucks over, as well as to provide a flat die for the non-grooved side.

With a 21 x 21 inch platen, you have 441 square inches to work with. If your vacuum pulls, say, 12 PSI, that amounts to over two tons of suckage---the vacuum can pull a plate down much harder than you can press down with the weight of anything you'd want to lift onto the platen.

So if you put a rigid plate over the plastic, and have a compressible gasket around the edge, your vacuum will suck the plate down with a couple of tons of force. If the PETG is what it rests on, it will press the PETG down with two tons of force, about three times what you'd get if over a 12" square sheet alone.

You can probably use regular weatherstripping from the hardware store as a gasket---say, 3/8" x 3/4" sticky-on-one-side foam rubber weatherstripping. If that's not tall enough to meet the plate, put a strip of something down and put the weatherstrip down on top of it.

If there's any flex in your "rigid" plate, it'll tend to push the plastic down around the edges, creating a seal (more or less) and squeeze the air toward the middle. That's the opposite of what you'd want. You'd rather it pushed down first in the middle, and squeezed the air out through the grooves, toward the edges. So you may want a plate that's slighty convex. To get that, you could put a few sheets of paper under the plate, of different widths, so that they add up to slightly thicker in the middle than toward the edges. If you put something slightly flexible under the paper, such as a sheet of acrylic, that will smooth out the stairsteps of the progressive thicknesses of paper, as well insulate the PETG from the thermal conductivity of the rigid plate.

Ideally you'd want a "rigid" plate that was very, very slightly flexible, so that this setup would push down hard in the middle, forming the PETG into the grooves there, but then slightly bending the plate so that it flattens and the pressure moves outward and squeezes the air out through the grooves, as the PETG is formed into the grooves in an outward wave.

The built-up-layers of paper in the middle and the flexure of the plate under two tons of pressure should both be about the same as the depth of the grooves you need to form.

12-09-2006, 12:45 AM
I want to absorb this slowly, You did not mention heat! Then this is pressure form, and if @ 12 lb I have over 2 tons then @ 25 lbs wow, I like it. Convex sounds logical. Wow, the insanity I've put myself through, to be given a possible answer that's really quite simplistic. Thanks Paul I'll give it a whirl in the morning

12-09-2006, 03:47 AM
I want to absorb this slowly, You did not mention heat! Then this is pressure form, and if @ 12 lb I have over 2 tons then @ 25 lbs wow, I like it. Convex sounds logical. Wow, the insanity I've put myself through, to be given a possible answer that's really quite simplistic. Thanks Paul I'll give it a whirl in the morning YES You need to heat the mold otherwise the mold will cool the plastic to fast Try to heat the mold as evenley as you can try to get it atleast 200 degrees (hot to the touch) Some one said to use a wire mesh under the mold .That will help ! screen works too.Im sorry if I'm repeating but dont have time to read the prier posts but will be glad to help you if you need me . From the pictures I would think you are trapping air being that flat .Heat will help that but you might have to drill some tiny bleed holes

12-09-2006, 03:53 AM
Next week I have to do some forming and if I have time I'll make a short movie showing you how to do it . Vacuum forming is not something you can learn over night but this might help you

12-09-2006, 10:47 AM
Hey Ringo, Yeah yeah yeah (beatles) Was it that obvious that I had never formed??? You ain't kidding I need help and what I'm getting and more would be greatly appreciated.

If the hole under the plate are covered would that give more or less pull? or is it the more holes the more pull?

12-09-2006, 03:51 PM
Ok I had a chance to read threw the tread The guys helping you are giving good advice!I'm still a little cornfused on what you are tying to accomplish .25Lbs of vacuum is fine but a 10 gallon tank might not produce enough volume. I don't know how big your pump is but bigger tanks require a bigger pump. I'm still in shock over what I paid for my last pump It must have pure gold parts inside it .LOL
Don't cover the holes under the mold. It will defeat using a screen under it. Are you not getting enough pull in the center of the mold? Heating the mold really plays a big part here! and also drilling tiny holes around the inner side of the frame (rim) of the mold can be a big help too. The fact your mold is not very tall makes it easier. You can attach two tanks together to incress the volume . My machine holds a 2'x4' sheat .The oven is on the top and the frame that holds the plastic is moved up and locked at the oven then I watch to see how far the plastic sags in the middle when heated .depending on the plastic used 2"to 4" of sag and it 's ready.then the frame comes down to the platen covering the mold as soon as the frame is down the vacuum starts pulling . From the time the plastic sags to finished form takes 3 or 4 seconds. If your vacuum is at zero your tank is to small . You should have Vacuum left in reserve after the plastic has formed

12-09-2006, 04:32 PM
If the seal around the platen edge is pretty good, it shouldn't take a big tank to do this.

A tank basically does two things:

1. It sucks most of the air out from under the platen in a hurry, getting the plastic to more or less form around the buck, and greatly reducing the volume of space between the plastic and the platen. (In doing so, the tank gets polluted by the air sucked in during the initial "pulldown." If the tank isn't several times bigger than the volume of air being sucked out, it will significantly weaken the vacuum in the tank. (Or you can use a 2-stage system where you use a small tank for that, shut it off, and open another tank that's not polluted.)

2. It sucks most of the remaining air out, pulling the plastic down really hard to get good detail, and stays ahead of air leaking in around the edge of the platen.

In this case, #1 is very easy. The mold is basically flat, so just draping the plastic over it will not trap a lot of air under the plastic, as it would with a tall, skinny buck.

So what the (pump and) tank need to do is mainly to stay ahead of leaks, to build up a good vacuum under the plastic---or upper plate, if you go that route---and hold it until the plastic has cooled below its thermoforming range.

If there's a lot of leakage around the edge of the platen, a small tank will fill up with air fairly rapidly, weakening the vacuum substantially before the plastic has cooled below its thermoforming range. This will gradually release the pressure, maybe too soon.

So if you don't have a big tank, it's important to have a good seal. You can make a pretty good gasket out of weatherstrip by

1. mitering the corners without cutting all the way through it---cut a triangular notch on the inside edge where you want a corner, and bend the weatherstrip until it meets itself. Glue that seam shut with some kind of rubbery glue. (Contact cement or silicone caulk.) And...

2. putting a little petrolum jelly on the top of the gasket to help it seal against the plastic or plate or frame or whatever it seals against.

12-09-2006, 06:06 PM
I tired the weather strip only, it kept getting stuck to the petg, I replaced it with some silicone rubber strips that I had laying around. I then siliconed them to the base. Tomorrow I will lay a piece of cold petg on the border of silicone strips, let the vac max out then see if I have any leaks.

I have some real fine 44x44 stainless steel mesh, I will open up the holes under the plate and place a piece of that down.

One of the things I noticed today was, that the sides of the [etg are not getting saggy, I tried a hot gun to little avail. I measured the heating unit, hot area is 16x18, that in itself makes a huge difference. The original hot plate for the unit had 39" heat rods in it, any idea where I can find them?

After and during the pull I usually have beween 10-15lbs of pressure or about 3 tons.

I didn't think of the mitering and have already cut the silicone rubber.

Looking forward to your version of a hard days nite Ringo

One of Many
12-10-2006, 01:28 AM
Forming temp will be around 250-320 deg.

A couple potential problems contributing to your dilema:

The terminology you use(Mother, Father plate and lens) is adding some confusuion to the process. The brass plate is a die, a mold or a tool, the material is just that.....PETG material being turned into a lens.

The material will NOT be an exact copy of the die due to shrinkage of roughly .007/inch. Which will equate to a slight pitch loss in your lens after cooling. Over all the part will end up .084" short compared to the die original dimension of 12" x 12". The materials extruded direction(AKA grain) can play a part in the shrink also. Shrinking more in one direction than the other. While it may not be a true dimensional copy, it will pick up any surface imperfections.

Of course, I do not really know if you need a specific feature/detail outside of the top ribbed surface besides a flat ribbed lens after forming.

Given a glimps of the equipment being used. If the top surface was my only concern. I would support the die 1" above the frame it is setting in now(vacuum box riser?), and add the ring(or incorporate it into the riser) as I posted previously. I could get away with using a much smaller piece of material(if I had an adjustable frame). Maybe 15x15 or 16x16 which includes 1/2-3/4" all the way around to clamp on. The frame will absorb some of the radiant heat, so some excess is required since that area will not be quite as hot.

The holes in the plywood deck, outside of the die do not serve much purpose unless you truely need material over each end of the die to hang below the finished ribbed portion. I am confident raising the die, installing the ring, sealing off the vacuum to only the area within the ring, and letting that hot material seal on the added ring will yield a formable part within the ribbed die surface. The mesh under the die allows the vacuum free flow. Although as the material pulls into the .007 grooves nearer the ring, won't get perfectly tight. The parts trim line should be inside this anyways. An oversize blank of material is no big deal. If you duct tape where the die sets on the plywood, that would be good enough.

Informing you of the extruded lenticular sheet was just an idea that you might not need to form your own at all.

Yes, even as Ringo posts. The die must be near forming temp. You do not want the material to freeze in place on first contact with the die BEFORE it has a chance to form. He knows that gaining the experience to READ the reaction of the material as it forms(or in this case doesn't form) does take time.


12-10-2006, 08:55 AM
Morning All,
DC, I like your piece on shrinkage as there are two parts to my process that I will be Forming for! One part the shrinkage will not be a problem. this is the part we are discussing now, I will look for an adjustable clamp, I would like to reduce size petg needed.

Taking into consideration all the great info supplied, I am going to:
a. Replace my multi hole base to the single hole.
b. As my mesh is very fine! 2 layers to a point 1/4" around the edge of my plate.
c. Bring in my silicone rubber ring to 1/4" from the mold.
d. Miter corners
e. Keep the lifters (aluminium strips) at either end of the parallel lines.
f. Raise the plate above the frame 1".
g. Lay a board on the ring and do a leakage test, I guess the best way is the wet finger method.
h. Hotgun the die.

The Second part, where I need to be as precise as possible. Correct me if I am wrong, I will use the hot platen method, where the petg is is pressed into the die. My theory being the longer the die and petg are one (petg sticks to the die), as they cool down to the lowest temperature (unknown at the moment) then the shrinkage will be less?

Does anyone know of a conversion or attachment for a reduceable clamp.

Have a great day all, GAry

12-10-2006, 10:10 AM
I'm not sure what you mean by "plate."

Are you talking about the lower die, which forms the grooves, or an upper plate like I was suggesting?

If you use an oversize upper plate, and have a nice piece of silicone to use as a gasket, you don't need to miter the corners. (It would be a shame to cut a nice piece of silicone, and silicone is hard to glue together once you do.) Just round the corners of the gasket if the silicone is flexible enough. You'll lose a few square inches of area under vacuum, but that won't be a big deal.

12-10-2006, 11:12 AM
Actually I am going to try both ways,
a. as you mentioned! I have a noce pc of slightly convex 1/2" plywood.
b. Heat up the die to approx 200/220f
c. Heat the plywood.
d. lay the petg on the die.
e. suck like crazy.

a. Heat up the die to approx 200/220f
b. heat up the petg and bring that down.

Prior to both applications I will reset up the area as mentioned in the earlier thread.

Pic from the studio, ah it's a beautiful day hear, mind you I'd rather be in the mountains camping.

12-10-2006, 02:25 PM
Just looking at your pictures . When the frame that holds the plastic is down and you are looking at it horizontally the platen should be higher also if the corners of the platen are rounded helps too I built a platen while back using a wood base on top I laid a heavy wire mesh and then covered that with aluminum sheating using a rubber mallet I slowley worked the aluminum around the sides and nailing it on the bottom .I used a good silicone around the edge of the aluminum to seal it the corners are rounded so it takes a little time . drill the holes in the aluminum not the wood. the wire mesh provides the space for the vacuum then I drilled two holes 1 1/4" threw the wood on the bottom of the wood to attach to the tanks The frame is steel and is lined with fine sand paper .when the frame is down the outer edge of the platen seals great .I suck at explaining things but I'll take some pictures. One last thing is to preheat every thing including the frame! when you are forming.230 degrees is not hot enough you need to be closer to 300 on the plastic .The mold should be around 200

12-10-2006, 10:23 PM
I took the unit apart and rebuilt it, (pics) I will speak with temperature controller tech tomorrow and get that on line. Do you think I need to raise the die any higher or does anyone see any immediate changes. the base board is 1/2' higher than the clamp.

Air pressure very good holds at 25lb for about 2 mins and then very very slowly recedes.

One of Many
12-11-2006, 12:01 AM
25lb or 25 inches of mercury(hg)?

If the ambient pressure acting on your skin is 14-15psi and 0psi is 29-30 inches of mercury. We can remove the ambient pressure on one side of the plastic(vacuum) to let the 14-15psi on the opposite side to do the work for us.

With that in mind. Consider what will happen during forming with the void under those thin aluminum plates.

How big is the hole under the die to port vacuum?

A 1" hole or several 1/4" holes should work fine. The screen under the die need not be a full sheet. I've used a few 5/16" flat washers to do the same thing. Strips of aluminum, whatever.....Just enough to support the die and leave room for vac flow.

Since you won't/can't use a total sealing ring up close to the die. At minimum I would trim the shim you have under the die to net width along each side and double back tape some solid bars to the plywood deck(same height as the die top) in place of the thin side sheets. The vacuum will still pull between the bar and the die, but support the material enough to keep from sealing off the grooves until fully formed.

The silicon rubber strips seem to me to be more of a nuisance than any asset to the forming process. Again unless you need some feature or detail inside the space between the silicon and the die, I am clueless how it will benefit the part. The material outside of the strips will not fully form and will be scrap anyways.

That large of a blank (" x " sheet of .02 PETG) may sag around 4"+ at forming temp. For smaller blanks, you would need to fab your own sub frame pieces to fit into the master frame you already have.

If you have not already seen these. Here is a link to some video's of actual forming on a machine similar to yours.

Vacuum Forming How-to Video's (http://www.imperial-armor.com/videos.html)


12-11-2006, 04:56 AM
A Shop vac??? yikes LOL Actually My first little formen used one of those a wopping 8 LBS. His frame looks good is yours like that? I agree with One on the silicon strips you shouldn't need them That's one reason to have the platen up a little higher then the frame so it seals around the out side edge of the platen itself. It looks like the guy in the movie had his heating elements spread out evenly (a good thing) but if he had stainless of even galvanized sheet behind the elements the heat exchange would be a lot better and also hotter.Dont purge the tank like he did in the movie. Time it at frame down just before or just after. If you do it to soon you will lose that hard pull . The tank pulls hardest when you first open it .With a vacuum cleaner it dosen't matter (no tank ) but for what you are doing that wont cut it.. When you first heat the plastic it will wrinkle a little and even sag then it will tighten up and then sag again .That's when you watch how far the midle sags and when it get's to that point it's ready to form If you do it on the first sag the plastic isn't near hot enough if you get it to hot then you can get webbing or thin sides from stretching. I dont really spell this bad two weeks ago I spilled a soda on my keyboard That and I need to proof read before I post sorry bout that

12-11-2006, 07:27 AM
I don't really understand the setup in the pictures. What kind of machine is that?

Is there a regular platen under the plywood sheet? Can you use it?

It looks like a commercial machine. Most commercial machines either have a fixed, raised platen on top, with a rounded metal edge that the hot plastic forms around and seals to directly, or they have an open-topped box with a platen that can be raised and lowered inside the box. (And the plastic seals to the edge of the box.)

If you don't have either, I can see why you'd jury-rig a sheet of plywood as a platen. And if so, I can see why you'd need the silicone gasket---the plastic isn't going to seal very well to the rough edge of a sheet of plywood, as it would to a rounded metal platen edge.

Please do explain what's going on. What's under there?

Also, where is the heater? Is the green thing stacked on top the heater, or just a lid, or what?

The angled frame looks like the hinged frame for a flip-over former, where the oven and platen sit next to each other; you heat the plastic over an oven, then flip it over onto the platen. But there doesn't seem to be an oven on the other side of the hinge, unless the plate sticking up in back is part of a funny hinge system... so I'm not at all sure how the machine is supposed to work.

Could you post some pictures that show the whole machine, and explain its usual operation?

12-11-2006, 11:31 AM
The machine is an Auto-vac, The way it works is
a. The brass die rests on the stationary base under the die is the mesh
b. The base has a 1 1/4" hole in it.
c. yes the heater is the green thing above the base.
d. The angled frame is the clamp, it goes up to the heater then slams down over the die.
e. yes, the silicone square around the die is due to the fact that my heater is 17x17 and the base is 20.5x20.5.

The process I am useing is,
a. Heat die, by manually resting it under the heater.
b. Placing die in shown location,
d. placing a pc of petg in the clamp
e. Raising the clamp under heater.
f. dropping hot plastic (clamp unit) over die.
g. Vacuum plastic.

You all mentioned the silicone strips, as i mentioned above they are there to reduce the vacuum area, the petg does not get heated up on the edges.

The two strips at either end of the die are to hold open the valleys, so that the petg does not seal on the edges of the die.

Not shown is the fact that the clamp hold the petg 1/2" below the base.
The vacuum holds well at 25? for a couple of minutes then, very slowly recedes.

i could not find a pic defintion of the steel ring effect is it possible that is what I am creating with the silicone border?

The petg does sag and sometime ends up spidering, I presume this is caused by too much sag?

One of Many
12-11-2006, 01:44 PM
The machine is an Auto-vac, The way it works is
a. The brass die rests on the stationary base under the die is the mesh
b. The base has a 1 1/4" hole in it.
c. yes the heater is the green thing above the base.
d. The angled frame is the clamp, it goes up to the heater then slams down over the die.
e. yes, the silicone square around the die is due to the fact that my heater is 17x17 and the base is 20.5x20.5.

The process I am useing is,
a. Heat die, by manually resting it under the heater.
b. Placing die in shown location,
d. placing a pc of petg in the clamp
e. Raising the clamp under heater.
f. dropping hot plastic (clamp unit) over die.
g. Vacuum plastic.

If the blank is larger than the heat area, that is not going to help a'tall.

You all mentioned the silicone strips, as i mentioned above they are there to reduce the vacuum area, the petg does not get heated up on the edges.

The two strips at either end of the die are to hold open the valleys, so that the petg does not seal on the edges of the die.

Hence my recommendation to use a ring up close around the die to further reduce the area of vacuum and give the material something to seal against other than the ribs at the edge of the die. Duct taping the ring at the plywood deck will seal off the outside world too.

Not shown is the fact that the clamp hold the petg 1/2" below the base.
The vacuum holds well at 25? for a couple of minutes then, very slowly recedes.

i could not find a pic defintion of the steel ring effect is it possible that is what I am creating with the silicone border?

The petg does sag and sometime ends up spidering, I presume this is caused by too much sag?

If the outer material is not hot and plyable, it won't seal on the outer edge and may not even allow the frame to drop completely into position.

The term spidering should be "Webbing" where the material pinches against itself. Yes, it may be to hot or your blank is to large and the die cannot eat up the sag in the material. The ratio of heat over time can be easily mis-understood. Having it heat up rapidly may not be the best approach. You may need less heat but more time to soak it to a formable state to get a better heat uniformity.


12-11-2006, 03:55 PM
Your machine looks like one of mine and frame going straight down is a great set up.It seams like a nice machine but I too don't understand the wood platen and that might be a lot of your problem if you would like I can show you how to build one pretty cheap that would work a whole lot better. You are never going to get the results you want if it dosn't seal around the platen just wood is not a good choice here.Also if the edges are not getting heat that causes problems too and could contribute to it not sealing and even throw you off on the saging .Hot in the center and cool on the outside = bad seal and Webbing,hammers flying ,broken windows ,spilled beer ,cat's and dogs living together

12-11-2006, 09:35 PM
I finally got the temperature controller to work, zapped up the heat to 350f and that's when bad seal and Webbing,hammers flying ,broken windows ,spilled beer ,cat's and dogs living together etc I found that the heater is cool in the back. Tomorrow i will take a ride up to Danbury conn. and pick up somr 21" rods for the original heat hood that came with the unit.

The wooden platform is just that! it raises the petg above the clamp by about 1/2" so that the petg is pulled down. If I take that out the petg wibe about an inch up from the metal base of the unit, which is better?

The ring, should it be parallel or higher than the die if higher how much.

thanks guys GAry

One of Many
12-12-2006, 12:47 AM
Spilled Beer?:eek: I hope the dog and cat got some and it didn't go to waste......unless the floor needed a swabbing anyways!:D

The ring should be at least the same height as the top of your ribs on the die. Maybe a tad bit higher wont hurt. It does not need to be anything fancy. I've gotten away with MDF smeared with silicon sealer. The silicon keeps the hot PETG from sticking to the wood and tearing it up. You could screw it down with drywall screws to the plywood deck. Metal would last longer, but do what you can get away with for a trial run just to prove if the forming produces a quality lens.

I have pulled .01 petg over a .025" thick 6" scale on a simple aluminum drilled deck plate and you can read the numbers and the 64ths lines in the petg. Not that it is good for anything else, other than to show the kind of detail thermal forming was capable of. With the shrinkage it's off just a wee bit(.04 in 6")!


12-12-2006, 02:00 AM
Ok Thats a decent machine I checked the site. and it will not perform as good with out a decent platen If you dont do this where just gonna have to go out side and settle this !!!! Dangnabitt LOL Parts list:1)Aluminum sheet 2 pieces Enough to cover.Top,Sides,and a little over lip on the bottom of your platen ,and one to cover the top of platen It should be between .040 and .080 with a alloy like 3003 -5052 2) 1/2" mesh screen, enough to cover the top of the platen .3)Box of nails Small1/2", 4)2 tubes of silicone 5) contact cement, 6)platen wood you can use FLAT particle board it works fine 3/4" and the size of your platen and box the bottom with 2x2s, 7)get 4 or 5 1/16th drill bits (HSS) and what ever fitting you machine uses to attach the inlet.And two sixers of Bud auta do it. Stolen from frends fridg.(drilling the holes a inch apart sucks) Tomorrow I will tell you how to build it(chair) PS 350 is too hot bring it down to 300 is a better starting point.

12-13-2006, 11:00 AM
Good morning all,
Well it was a really great trip to conneticut yesterday, This bloke Tony silva at high temp products, was a real salt of the earth type! He didn't know me from a hole in the wall, and got his crew together and built this heat unit into the old hood. (pic)

I am in the process of installing it. I am taking the new temperature controller with thermo rod and attaching it to the housing ( the old system was rusted and burnt out.

What do you thing? she looks great with the old hood eh. i'm going to look for aluminium sheet, it make a lot of sense as it will help heat up and keep the heat in the die, i have plenty of silicone and look forward to seeing your plans Ringo.

I am also going to set up the ring that DC was talking about only I am goin to use the silicone rubber and make it a little higher than the die, this way I can also try out Pauls idea with the slightly conves board.

I gotta run as there is a lot to do, will keep you guys updated.


One of Many
12-13-2006, 01:35 PM
All of the heat controllers we used were basic percentage timers. Med was 50% on, 50% off. No actual temp display has ever been used in the equipment. Of course these guys could form plastic in their sleep.

The Mercury contactors can be great. We had some that are originals going on 35years old, and others that we'd be lucky to get a couple of years out of. All in the same load applications.

The new elements looks good. Probably a good idea on keeping the silcon ring a bit higher than the die. I hope it does not compress under vacuum and seal off next to the die. I don't know what the heat might do to it. It should be good to 450-500deg as a guess.

Here is wishing you luck!:cheers:


12-13-2006, 03:12 PM
Silicon is usually good to over 1000deg f.

I looked at their web sight and they sell allot of different types of elements, which one did you get and what was the price?

12-13-2006, 07:10 PM
Finding time has been hard latley but Ill get it for you

12-13-2006, 10:13 PM
I took in a 21"x21" shell, They gave me one 2,000 watt/220 volts, 1 phase/9.1 ampsceramic fiber heating panel with embedded K-A1 coil element. for $600.00. And let me tell you! That baby cooks, It smoked like crazy for about 30 mins. and now is even consistant heat.

Ringo, I didn't understand the picture, I'm sure we'll get a clearer version when you have time.

DC The silicone I have is quite hard approx 50 durometer A, but your point is very valid, I am now going to use silicone coated wood strips and save the high silicone for Pauls idea with the pressure plate.

When I left today the silicone was drying under the aluminium sheet, Looking forward to trying it out tomorrow. I'll let you lknow how it goes.

One of Many
12-14-2006, 06:19 PM
Ringo is showing how to create a plenumn of your lower platten to replace the plywood deck you have now.

1 is the top skin with an array of 1/16" holes over the entire top of the sheet.
2 is the 1/2" screen
3 is the lower skin
4 is the base plywood deck.

I fab'd similar types of these bases for permanent mounting of evey tool I made. I used 3/4" marine grade 12ply wood, but I didn't use the lower aluminum skin. I smeared the top of the ply with a thin coat of silicon to seal it. Set the screen on that silcon, then screwed down a 3/16-1/4" plate of aluminum sheet as a top deck with a bead of silicon sealer around the edge. Then I would smear silcon that squeezed out around the outside flush with the aluminum deck. After the silicon cured, I would orbital sand the deck and round off all sharp edges. Sometimes I'd sand blast it with very course sand.

On the bottom side of the plenumns ply, I would seal and mount a standard cast iron flanged pipe fitting bung for the vacuum line connenction. The bung hole through the ply could be 1/2-2"...... but....(no pun intented:D ) the aluminum deck could have any configuration of vac holes in an array for prototypes as Ringo depicts or 1 hole under each die mounted permanently. Let the job direct you as to what, where and when on the hole configuration on top of the platten plenumn.

I was never a fan of the prototype plattens with loose dies for more than a few pulls. Knocking out the dies could make them go airborn. If your platten or your frame moves vertically, having the dies permanently bolted to the deck and using air eject was better than beating and prying them out of cooled plastic. Depends how fancy you want to get. A rear hinged frame does not let the material pull straight up, so taller permanent mounted dies may not work for this application.


12-14-2006, 08:11 PM
DC how do you read that in this pic? I've been looking at it for a while now and still can't make head nor tail of it. nothing personal Ringo. I guess you blokes have been doing this so long you can read into it. But, for me being new it's kind'a confusing.

One of Many
12-14-2006, 11:51 PM
I really should not have answered for Ringo........but, this is what I read into his plan. And, admittedly I was a little over eager to help out. Shame on me...shame shame sha.......ya whatever!

Also......Ringo explained his method in post #30, prior to posting the marked up pic. That is how I translated the pic as a sandwich of the parts listed. The silver pillow with the freckles wasnt a dead give away?:stickpoke

The reason for the alloys he had selected is to allow forming the top sheet around the edges with a mallet and finish off with silicon sealer and a vac connection. The only thing I left out was the 2x2's, but I was not quite sure he was really creating a plenumn that thick or he intended those for structure under the sheet. He did not mention a top and bottom sheet of MDF, but he did mention a boxed in contruction(I. E. Plenumn platten?).

I'd avoid the bare MDF, as it can pull vacuum through it unless sealed. Melamine coated MDF is nice stuff to work with.

Again, I'll take a wedgie and sulk in a corner, if I am out of line with Ringo.(wedge):D


One of Many
12-15-2006, 12:42 AM
I hope TallPaul puts his posts back up. He had a number of good images near the same setup as Ringo's description. I recieved the email responses he posted, but he must have deleted them here. He made a good point about the deck height slightly above the frames plastic clamping position for a good seal during forming. Nice little former too!


12-15-2006, 05:10 AM
I tried to post 3 times and it deletes itself. I don't know what happened to my previous posts. It seems to work now so lets try again.

Here's some pics of my home made machine.




The platen is 4 pieces and is above the clamp frame. The plastic seals on the edge with 0 leaks.

12-15-2006, 05:51 AM
Bung Hole??? LOL You are not out of line at all ONE .Screnzzz. I'm Sorry Taking so long .My free time bolted on me. I'll be back as soon as I can but One pretty much nailed what I was showing you. I have to make some molds for shields for my mill and I'll tape Forming it for you .

12-15-2006, 11:47 AM
Great pics tall, wow you really took pride in that piece, nice wood. makes mine look like something from a junk yard, But, I put a lot of love in her.

More pics. The peice of aluminium that i cut out of a freezer door (1/8") was not wide enough to drape over the edges, it is nailed down with silicone under and around the edges. The edges and sides I rounded off.

The hood fits snugly over the clamp.

I'm approx 1/2" above the bottom of the open clamp.

I did not make hole all over as I had already sealed the aluminium to the platern, there is a 1 1/2" hole under the sheet.

The ring is approx 3/8" above the die, I rubbed it with silicone.

If this does not work I will try the same thickness silicone rubber with the convex board

The temperature controller I believe is important as the heat unit installed can float up easily to 2000f.

i am waiting for the silicone to dry, then I'll start her up.

Also, one of the difficult and time consuming things has been bringing the die up to temperatureit's 5/8" brass, So, i am placing it under the heat transfer machine with the platen down, takes about 5 mins.

12-15-2006, 12:29 PM
Waaaa Bammmm There it is Tall Pauls Platen is a good example Screnzz. Nice and flat ,uniformed holes, rounded edges, fits nicely inside of frame

12-15-2006, 12:45 PM
I thought the platen design discussion was worth its own thread, so I started one:


The first post gives a link to an article I just posted on hobbymolding.com.

12-15-2006, 01:34 PM
nice, easy to follow I'll absorb it tonight when get home. I have a question or 2 on trouble shooting the process of vacuum forming. I believe my sytem is now set up pretty good, I took her for her first run, I wanted to pull my hair out!
.020 petg sides not softening very well at all, the centre looked like a 9 month pregnant woman, It also looked like it was going through a crystellene process.

is this a heat thing, like It's to hot

12-15-2006, 03:23 PM
nice, easy to follow I'll absorb it tonight when get home. I have a question or 2 on trouble shooting the process of vacuum forming. I believe my sytem is now set up pretty good, I took her for her first run, I wanted to pull my hair out!
.020 petg sides not softening very well at all, the centre looked like a 9 month pregnant woman, It also looked like it was going through a crystellene process.

is this a heat thing, like It's to hot
Sounds like a heating element problem Not getting heat to the sides and to much in the center. How far are you letting it sag? ((((How much time untill it sags)))) .A temp gun would be a big help here .Are you preheating including the frame? Crystellele << Never seen that happen. Using a heat gun around the sides while it's heating will help but a band aid at most .If the plastic is not heated evenly you will fight it. Remember the plastic when first heated will wrinkle a little then sag a little then tighten smooth .At that point is when it is soft and you are a few seconds away from pulling .Wait untill it sags .There should be NO wrinkles anywhere on the plastic at that point and if you where to touch close to the side it will feel soft. A good Heat gun is nice to have on hand .A lot of times while the plastic is heating I'll use it to keep the molds up to temp. Or if sometimes I screw up and not have enough vacuum do to not shutting the valve all the way or not waiting long enough . A heat gun has saved my arss by using it right after the pull

12-15-2006, 04:19 PM
I'm guessing it's way too hot and you're getting tiny bubbles that make it look like it's frosting up. (Not being able to see what it actually looks like, that might be a very bad guess.)

PETG's thermoforming temp is low.

How long is it taking to heat up enough to sag? To seriously sag?

Correct me if I'm wrong, but you're using a big heater relative to the size of your plastic, right? You shouldn't have big evenness problems if your plastic is in the right range of distances from the heating elements.

(Ideally, significantly further than the spacing between heating elements, but preferably smaller than the distance the heater extends beyond the plastic.)

Putting it further away than the heating element spacing allows the hotspots from the individual heating elements to spread out and overlap before they hit the plastic. Putting it closer than the overhang distance insures that the whole sheet is getting IR from all sides. (You can also get that effect without much overhang, if you have straight reflective side walls down to the plastic.)

12-15-2006, 06:33 PM
I might have missed it but could you take a picture under your oven so we can see it?

12-15-2006, 08:18 PM
Theres a pic on thread 42 Ringo, After a review of whats happening it seems the Heater is like the ever ready rabbit, it just keeps going and going. The bloody 2nd controller is wiggin out. it tells the temperature only, the system is not controlling it.

It seems logical to me that, if the temperature is in the 400 mark, that it will be so hot that the centre will sag big time before the sides soften, and as dr says little bubbles, yes they could definitly be considered little bubbles.

The petg size is 22.5x22.5 platen is 20" heater is 20".

The distance from the heating elements to the petg is 1.5", with a space between elements of 1''.

I didn't time the sag will do that tomorrow, I'll also take a pic of it.

Remember the plastic when first heated will wrinkle a little then sag a little then tighten smooth At that point I still had a couple of inches all round that were still hard.

As a last case scenario I'll use the heat gun only, I would like to see if I can get it to work without it first.

12-16-2006, 12:33 AM
I swear I'm gonna kill my PC I did this big reply ,hit spell check and poof!!! Gone. Don't use a heat gun for your main heating source Get more room between the oven and plastic !1.5 is to close and explains what it is doing. Find a way to move the oven Higher around 5 inches total is a good starting point .Different elements have different patterns so you are going to have to experiment on the height before bolting or welding. I know you are getting frustrated but trust me when you make your first good pull you will do a happy dance. Just gota get the bugs out. I don't use PETG that much but I think If you don't stretch it to far or poke any holes you can reuse it Don't try to pull it if it's not heated properly If it does the same thing it has been add more space (oven to plastic)

12-16-2006, 10:11 AM
Thanks for for the positive attitude, I'll put on some yee ha dance music and pratice a few steps. Being that my up and down is air assisted and my heat hood is a constant I will go to manual and use a couple of blocks.

Also, as I have an exhaust ean close by I'll cover it up so that it doesn' pull air from under the heat panel. do you think I should attach a foil skirt around the hood?

Ah, it's another day in paradise! i couldn't locate a temperature gauge to double check consistancy of temperature under the hood, will pop over to graingers on Monday.

12-16-2006, 12:34 PM
The room your in will be like a tanning bed if you don't use some kind of cover around the oven that and the temp will be inconsistent .And you don't want that heat loss. Foil should work while your experimenting with oven height but once you get close build something more permanent .Steel sheeting .Galvanized will work stainless would be better. then build some sort of housing around the out side to help insulate it. I hope I'm not confusing you I'm not the best at explaining things .One,DC,To tall,All are giving you good advise. I have read threw some of there threads and they are pretty impressive . One last thing you should add is some kind of switch to shut the oven off when the frame is in the down position

12-16-2006, 05:43 PM
I with you on the heat loss, My NEW dilema is as you can see in the pics my unit wasn't made for a skirt around the heat unit (maybe it's a boy) also the old heat rods were two inches into the unit, the new heat system is flush. I will experiment with the height Monday, after I 've finished with the tec for the temp controller.

Getting back to the skirt, I will have to bend the back board to allow a skirt to hang, which would be okay as it would help shield the electric box.

I can set up an on off system so that when the hood comes forward it will turn the power off, then as it back it will switch it back on. Great

I agree One, dc, and Totall had a lot of great advise, but, so did you mate! It just took me a while to understand the logic and language. All you blokes have been bloody fantasic and I would have been at a loss without you all, so thanks for being there.

Catch you next week

12-16-2006, 06:38 PM
Plain aluminum flashing works great for reflective sidewalls, and it's dirt cheap. (Something like $7 for a roll ten inches wide and 10 feet long, or $15 for a roll 20 inches wide and 10 feet long. It may be called "valley roll" in the roofing dept. of your hardware store.)

Aluminum is more reflective than steel in the infrared. Flashing is not glassy smooth, but it's smooth enough that you get blurry "mirror images" of the heating elements that make it appear to the plastic that the heating elements continue indefinitely past the walls.

Just don't bash the stuff around... it's thin. (And don't cut yourself on it.)

12-16-2006, 06:43 PM
BTW, if you have reflective sidewalls, you can't control the heat by varying the distance of the plastic from the heating elements. If you move the plastic down, the IR that would miss the plastic without a reflective wall in the way just hits a sidewall, mostly bounces off, and still hits the plastic on the bounce.

If you don't have reflective sidewalls, controlling the heat by moving the plastic away from the heating elements is still a bad idea. It changes the heat distribution, so you heat the plastic unevenly.

To properly control heat, you need to control the heating elements, either reducing the voltage or turning them off part of the time.

12-17-2006, 02:57 AM
Actually guys you are both correct! I %$%%$* screwed up.

By me getting the new heat unit installed in the hood, I've taken away the area for the heat to mix.

The bottom pic shows how the heat elements radiate, by taking away the couple of inches under the elements, by placing the new ones flush there is no mixing area!

So, sides are needed to recreate the heat mixing area, I think what you are both saying is that, by finding a comfortable heat mixing distance with the use of aluminium foil, then replace it with a permanent mini skirt.

The other very important factor is temperature control. That I will resolve Monday.

My choices here are:

a. Replace heat element with original type.
b. Change height for air lift, make a skirt and cut away part of the back board, shoot!

Thanks guys

12-19-2006, 02:02 AM
Dr. Brought up a good point about oven height I wasn't so much suggesting changing the oven height to change the temp. By experimenting with oven height temperature isn't the major concern . What you are accomplishing is finding the sweet spot for you elements to be efficient in giving a uniform heat to the plastic. You already know what happens when you are to close . You will also notice when you are to far away .It will give strip gaps threw out the sheet You are looking for just before the floor of the element pattern .But don't go crazy on this just start at 4 or 5 inches and try moving it up and down and see how it's heating the plastic so you can see when it heats it evenly (No hard spots) .. To be honest My Machines all have side skirts I have never tried with out them Safety tip make sure your wires are shielded from the oven

12-19-2006, 02:11 AM
I'm not a big fan of having the oven moving that much while forming on a small machine I have never used one like that but I would think it would be kinda dangerous if it broke loose . You have another choice making the oven fixed and having the frame move up to it then down to the platen .Just thinking out loud here

One of Many
12-19-2006, 12:36 PM
Sorry for dropping out for a while. Power has been out for 4 days and last report was maybe by the end of this week.

I'll catch up and then post more suggestions if I can.

For now you are in well experienced company and gaining a wealth of your own!


12-19-2006, 01:01 PM
Hey there One, sorry to hear about the power, You were missed! Hope your power people get it together asap.

Ringo The combination of your response and dr's painted a very clear picture for meand what I got was basically what you just described, sometimes it's not what you read but how you read it. I definitly needed that combo of opinions to see both sides of the coin.

I don't think the oven will break loose, only, I am goin to try as much as possible to keep it more in the stationary forward position. The reason being more of your last comment (Safety tip make sure your wires are shielded from the oven).

I have just rec'd new temp controller, will let you know how it works out.
Have a g'day mate

12-20-2006, 02:29 PM
Hey All hows the power situation up in your neck of the woods, I hope that they've been able to fix it.
well guys she works good, consistant temp control, within a couple of degrees, suck like a __________ I'll let you fill that in. I took your suggestion all and used thin washers undeer the brass die. I going to pick up a sheet of aluminium for the sides of the hood tomorrow, as per dc and Ringo. i also have a couple of rolls of soft pvc coming in, any of you blokes worked with it?
Merry christmas everyone

One of Many
12-20-2006, 04:06 PM
Power is still out and reports of another wind storm are being tossed around so to speak. I guess rural living has it's down side, but nothing would make me move the 13 miles back into the city where I grew up. Several stores were looted when the power went out. Yet the city officials cannot make the connection to all that section 8 housing and massive apartment complexes they allowed to be built and destined to turn a neat little town into a rough neck slum!

One snippet I caught in your description of the plastic going crystalline?

I am not sure what you mean there. If you are getting bubbles, this could be moisture boiling in the sheet during heating. I do not recall if PETG was a hygroscopic material, but a couple days in 130-150deg heat box should take care of that. Many materials can retain moisture and that moisture will come out slowly at low temps or attempt to come out rapidly at high temps.

The thin washers do not just support the die. Where I had just put washers at the corners of large dies, the plywood would flex enough around the bung hole to seal off all vacuum to the die. The parts would form about half way then stop cold. A few more washers(siliconed in place) to keep the tool and the ply seperate was pretty simple.


12-20-2006, 04:51 PM
One snippet I caught in your description of the plastic going crystalline?

I am not sure what you mean there. If you are getting bubbles, this could be moisture boiling in the sheet during heating. I do not recall if PETG was a hygroscopic material, but a couple days in 130-150deg heat box should take care of that.

My impression is that PETG is slightly hygroscopic, but so little that it doesn't need pre-drying like (say) acrylic---usually. YMMV, etc.

My first guess would be heating it too fast, so that something in it vaporizes and bubbles rather than staying in solution or outgassing slowly.

I'd turn the heat down (if only by flipping the power on and off) until it takes several minutes to heat the plastic. If it still bubbles, I'd try pre-drying it. But if you're set up for pre-drying, it couldn't hurt to do that right away.


12-20-2006, 08:41 PM
I run into moister problems once in a while using Lexan It will look like tiny air bubbles.I'm sure One and DC know more about PETG then I do But I kind of think having it that close to the elements would stripe between them and melt directly under the elements

12-20-2006, 09:11 PM
I'm going to try a experiment and wondered if you guys ever tried this . I dug out my Protoform cause it has the most open area so I can see . Any way I want to make a dome shape with out any mold at all. The Proforma has a frame that moves up to the oven and down to the platen .Instead of evaking I'm going to fill the tanks with Psi. air. I'm going to use a 2'X4' sheet of Lexan ,Heat it normally Maybe a little longer on the sag time and I'm not going to preheat the platen. but I am going to heat the middle of it with a heat gun My theory is with the platen cool around the edge it will seal fast while I blast the air out of the platen thus making one big air bubble .It has 2, 40 gallon tanks so it should be able to hold the plastic up until it cools (Reverse sag so to speak)making a big dome if all goes right! Plan B is to just let the plastic sag shut down the oven bring the frame down about half way and just letting it cool that way Which now that I think about it will try that first.Any thoughts ? Have you ever tried this or did I just have one beer to many when I came up with this

12-20-2006, 10:05 PM
My guess is you'd be better off just heating the plastic with the Protoform.

I've never done it, but my understanding is that you get pretty cool domes with acrylic by just sagging and/or sucking it into a circular hole in a box. You want the heat to be very even so that it sags into the same shape all the way around.

Not sure about doing it with polycarbonates, though.

If you're going for a circular dome, like a hemisphere, I'd think 2 x 4 sheets would be very wasteful. You could probably just make a 2 x 2 square box and attach the plastic right to it, if you put it up close enough to the heating elements. (You could even keep heating while it's sagging into the box.)

12-20-2006, 10:10 PM
So All, you've become a country boy eh, I'm somewhat envious, living closer to nature as anything else in life has it's ups and downs. Snow adds a real pretty look to the enviroment. a couple of years ago I went up into the Catskills in my suv with new tires, decided to see how good it was. I Drove in deep then slid into deeper snow, didn't get out till next day (I always have camping gear in the back), It was real nice getting up in the morning throwing on the coffee, Whoa this is a long story so I'll stop here.

Hey Guy's, I haven't seen bubbles lately, guess she doesn't like the heat! my presumption is bassically the same as dc.
a. No heat control.
b. 1" away from the heat unit.
I was waiting for the out side edges to soften, and the centre was halfway between glass and liquid.

For me the washers were to raise the brass die higher, the original mesh was very fine with very little lift. I thought a little more space would give a better
Whoosh of suction.

Coming from very little experience, also having seen some 10/12" domes, If one blows air up one will get a web effect. If one can blow heat into the inside of the dome, who know, you could probably blow yourself a place to go when the wife gets her knickers in a twist.

In my last thread I mentioned soft PVC! Anyone??

One of Many
12-20-2006, 11:20 PM
POWER.......has returned.......Hooray!

Funny that you mention that Ringo!.

Actually, I did do this with a piece of 4' square 1/4" lexan(polycarbonate). We just let it droop, but there was a lot of weight in that sag to help. It was a dome for a friends Hot Tub Gazebo. I built a lumber frame with shrinkage compensation of what I expected it to be as finished size with a flange for fastening it to the structure. Not like it needed to be precision, but it turned out very nice.

We did do bubble prestretch pretty often. If you have ever seen a snap back box forming, it is the opposite of this. The box opening is used as the seal edge. It is just not very practical to expect perfect dimensional repeatability if you are controlling the buldge manually with a gate valve. It works well for prestretch prior to actual forming on very tall dies. When the tall die is on the upper platten, it is timed such that the die pokes into the bubble and basically rolls the bubble on as the die descends. After the die base meets the snap back box, then the vac takes over to pull the material against the die. After a cooling cycle, the die and the snap back box retract from the material leaving the cooled part in the frame.

What are you making, a sky lights or cool bubble windows for a chevy van?:D


One of Many
12-21-2006, 12:07 AM
These are the foothills to the Cascades, so snow can be part of its charm, but also another seasonal inconvenience for commuting. I'll suffer with the Deer, Rabbits and the occasional Fox rather than dodge bullets in my old stomping grounds, Heheh!

DR had some good points on heat distance and distribution. As I have posted in another thread, you can string some wire lines to make a screen tray under your heater. With different screen densities, you can mask off heat in hot spots by placing the screen on top of the wire lines. This allows you some control over time, temp and radiant intensity, where you need it and where you don't. You are gaining some experience on how to read the materials condition. Let that guide you toward making the heat consistant throughout the sheet(I.E. Cause and Effect).

You can blow the bubble up, but keeping the air from leaking while it cools without it collapsing on itself could be a challenge.

We did a lot of PVC, but it was either natural ivory, black or gray. I would not say it was soft. The smell was awefull!
There was a keyboard cover/skin we did for a while, but I don't recall the material designation off hand. I want to say it was EVA or PVA or some thing rubbery.


12-21-2006, 02:10 AM
Boy talk about snow .We Are getting Blasted hasn't been this bad in a long time (Colorado) Any way what I'm building is a underneath cover for my tig mill so I do want it 2'x4' The travel on the X axis is closer to three feet I can trim it as needed. I hate making molds for just a few runs so I want to see if this will do it. I am a little curious to see how far it will stretch

12-21-2006, 04:19 PM
And Of course Thank you for the tips. I'll let you know what happens

12-21-2006, 07:52 PM
Wow it worked great! I just heated it and let it sag a little longer then normal.I'm inspired to make some skylights for my house. One of the pics is my driveway there are 2 cars under there somewhere. Sorry about the picture quality

One of Many
12-21-2006, 11:19 PM
Well, there ya go! Congrat's on getting what you expected.

You can keep the snow. Oooh, man that looks like Cold-orado!

And I thought I had it bad with the power out for a week. I hope you stocked up on beer before it hit. At least it won't get warm like mine did.:D

Keep your self safe and warm down there. Brrrrr!


12-28-2006, 08:03 PM
Wow Ringo, Looks like Franki blows a pretty good bubble, Way to go mate!
Good luck on surviving the snow, hope you have a snowmobile up there!
Hey Guys hope you all had a great Christmas season, and wish you a healthy New Year.